The present invention relates to apparatus and method for determining capacitance and resistance values in an electrical component and, more particularly, to an apparatus and method for determining capacitance and resistance values of an electrical sensor, such as a fuel level sensor, having both capacitive and resistive characteristics.
Various sensor applications use the change in capacitance of a sensor as an indication of a to-be-measured physical parameter. For example, in measuring the level of fuel in a fuel tank in an aircraft, a capacitive probe is positioned within the tank. The probe typically includes structure which defines capacitor plates or the functional equivalent with the dielectric constant between the plates determined by the level of the fuel in the tank as well as the air in the airspace above the fuel. The sensed capacitance is a function, in part, of the fuel level in the tank. In an optimal capacitive-type sensing system, the electrical resistance between the plates is very high, on the order of hundreds of megaohms, and normally does not influence the capacitive sensing function. However, under certain circumstances, abnormal conditions within the fuel tank can contribute to an unusually low resistance path between the capacitive plates. A low resistance condition can adversely affect the validity of the capacitance measurement. Such abnormal conditions include, for example, the presence of water in the fuel tank, contaminants that possess electrolytic properties, and microorganisms in the fuel. Thus, in sensing fuel levels using capacitive probes, it is also important to sense the resistance between the plates of the probe to determine the general conditions within the tank and provide an indication of those conditions which may adversely affect capacitance measurements.
The present invention provides improved timing at which the measurements are taken over Rubel et al. In Rubel et al., the sampling measurement occurs at the point in time in which the waveform is at a zero voltage value. In the apparatus of Rubel et al., failure to sample at exactly the zero voltage point results in a direct error in the measured value of the capacitance in the presence of any resistance. Such timing errors result from circuit path delay variations caused by component tolerances, time variations, and temperature variations of amplifiers and components. In Rubel et al., these timing errors remain uncorrected.
The present invention avoids the timing problems of Rubel et al. by employing a linearly varying ramp voltage which goes from one voltage potential through zero to the opposite polarity potential. The capacitance measurement is taken at approximately the zero-crossing of the excitation waveform to maximize the available dynamic range, but it is not a requirement of the invention that the measurement be taken exactly at zero as in Rubel et al. A correction factor provided in the self-calibration feature of the present invention corrects for any circuit errors. The present invention is stable under variations of components with respect to type, temperature, tolerances and time. This stability makes the present invention particularly flexible in the component requirements for its manufacture since any variation of the zero-crossing sample point is corrected for and has no affect on the final measured capacitance value.
Rubel et al. disclose a measurement technique which employs an open loop scheme, in which any variation in any of the circuit switching blocks results in a direct error in the measurement, and there is no system feedback loop. The apparatus of the present invention provides an accuracy of 0.25% without using precision components.
The present invention uses a closed loop technique which continuously self-calibrates the measurement circuitry as the measurements are made. Any offset to the measured values of capacitance annd resistance is monitored immediately before and after each measurement cycle. Standard components may be used to build the invention with no effect on long term accuracy or temperature stability.
Maier in U.S. Pat. No. 4,426,616 discloses a capacitance measurement system employing a capacitance reference. The present invention includes a resistance reference in a capacitance measuring circuit. A capacitor is sensitive to frequency which affects its signal amplitude and phase, while a resistor is not frequency sensitive. The phase of a resistor and capacitor are 90.degree. apart. The problems caused by the amplitude and phase differences resulting from using a resistor to calibrate a capacitance measuring circuit usually result in poor operating accuracy. The present invention solves the problem of amplitude and phase differences so that a readily available resistor of high stability and known value may be used as a reference for capacitance measuring circuit rather than a precision capacitor.
The present invention uses the same circuitry to measure both capacitance and resistance. The circuitry used is thus compact and combines one calibration reference for both resistance and capacitance measurements.
The present invention accomplishes resistance and capacitance measurements without any switches in the signal path.
The present invention uses no switches and accomplishes scaling by changing the rate of time-varying of the excitation interrogation waveform. The present invention is able to accurately operate and measure a wide range of electrical components simply by varying the interrogation waveform excitation ram rate. These changes are readily accomplished and allow the design flexibility to fuction correctly over a wide range of sensor conditions with no circuitry changes required.